|dc.description.abstracteng||With more than half of the world’s population already urbanised, global sustainability is partly hinged on how the urbanisation process is impacting the natural environment. Therefore, it is important to characterise, understand, and monitor the physical manifestation and interaction between the urbanisation process and environmental resources, in order to inform sustainable urban growth and land management strategies. Such information is especially important for urban regions in Asia and Africa, which have, in recent years, become hotspots of urbanisation. In many metropolitan regions of Africa for instance, a nexus of exponential built-up expansion, inadequate infrastructure, and complex land tenure has created heterogenous landscapes that are yet to be understood. One common feature within such landscapes is urban agriculture which is widespread in many African cities, as urbanites rely on it for their livelihood. To date, it is unclear how the resulting environmental effects of rapid urbanisation can be characterised and assessed. In addition, it is largely unknown how the soils that underlie the urban agricultural fields are responding to the sprawling urban trends. The aim of this thesis is to fill these gaps, by characterising the emergent urban landscapes that result from rapid urbanisation, and empirically demonstrate how soil fertility of urban agricultural fields is being shaped, using two typical West African metropolitan regions in Ghana. For this purpose, the thesis is divided into two main objectives as follows: (1) to examine the evolution and spatio-environmental impacts of rapid urbanisation in Accra and Kumasi, the two major metropolitan regions in Ghana, and (2) to assess the influence of rapid urbanisation on urban soil fertility properties of the Kumasi region.
To accomplish the first objective, we relied on optical remote sensing techniques. We used a support-vector-machine learning algorithm to classify retrospective Landsat satellite images spanning three decades from 1985 to 2019, in both Accra and Kumasi metropolitan regions. We then created a detailed land use transition map, and applied landscape and spatial metrics to characterise the emergent landscapes. To accomplish the second objective, we designed a novel grid-based soil sampling stratification that relied on the 1986 satellite image of Kumasi, and embarked on an extensive field campaign in the city, in August–September 2016. The sampling approach involved keeping factors that were not in the focus of the study constant (i.e. parent material, topography, soil type, crop and climate, while differentiating two urbanization categories. These categories were defined as: (1) long-term urban soils, which were areas that were under urbanization in 1986 (i.e. ≥ 30 years urban influence), and (2) short-term urban soils, which were areas that came under urbanization afterwards (<30 years of urban influence). A total of 636 top-soils (0–10 cm) were sampled, and consequently subjected to standard (ISO and DIN) soil preparation and analyses, such as soil organic matter (SOM), soil pH, total C and N, and effective cation exchange capacity (ECEC). In addition, we developed a novel sequential extraction method for assessing soil P fractions, as the inherent nature of tropical soils makes it especially relevant to examine the plant-available and recalcitrant P pools instead of its total contents.
We found that, for the first objective, built-up areas have substantially increased in the two metropolitan regions of Ghana at the expense of other environmental classes, leading to increased fragmented environments. In Ga West, a peri-urban district of Accra for instance, we found that built-up land increased >200% within the recent 30 years. Such fragmented environments underlie various urban environmental problems like floods and heatwaves. The analysis further indicated that the urban expansion process follows a general trend where the historical-core zones were initially hotspots for land transitions. In recent years however, settlements in the suburban and peripheral zones have expanded, becoming integrated into the conterminous urban areas of the metropolitan regions. The analysis also uncovered and highlighted a complex, unique, dynamic and multidirectional process, which showed that the urban-open-space class, being in a permanent state of flux, mediates transitions between built-up land and vegetation and vice versa. It was recognised that this process is central to the informal practices that characterise physical development in cities and towns in Ghana and other similar West African countries. For the second objective, the analyses demonstrated that urban arable soils that have been under long-term urbanisation influence have higher contents and stocks of SOM, plant-available and recalcitrant P, carbonates, exchangeable Ca2+, K+ and Na+, soil pH and ECEC, relative to their short-term counterparts. Both field observations and lab analysis confirmed that these results are attributable to the accumulation of household waste (degradable and non-degradable) and construction materials on the arable fields. In contrast, the stocks of exchangeable Mg2+ and total N were lower in the long-term compared to the short-term urban soils. We attributed this to the accumulated non-degradable materials, and most likely, to the increased intensity of agricultural use under urban land demand. These findings highlight a diametrically opposite consequence of urban waste management challenges on the wider environment in Kumasi. With these increased soil fertility indicators in the urban agricultural fields, which is an integral part of the urban-open-spaces class, we conclude that management efforts that seek to decrease the environmental impact of the urbanisation process can target this urban-open-spaces class, as it is the central piece of the land transitions in Ghana.
Based on these findings, we recommend that an integrative urban growth management strategy that brings together spatial planning and environmental resource governance, as well as an improved urban waste management system will be useful to enhance urban sustainability in Ghanaian cities. Given that the two metropolitan regions that were studied in this thesis are typical for West Africa, the findings of this research are also relevant to most other cities in the region.||de